It is known in the vehicle closure panel hemming art that roller hemmers can be used to hem the edges of metal, for example aluminum, automotive parts such as door panels, hood panels, and decklid panels.
Conventionally, roller hemming apparatus may be mounted to a multi-axis controllable robot and may include a hem roller carried by a support. The conventional hemming apparatus is adapted for hemming a bent portion of a workpiece, such as a door panel, positioned on a hemming die, by rotating the hem roller under pressure along the bent portion. The conventional roller type hemming apparatus can thus be used for continuous hemming along the contour of the edge of the workpiece.
In order to achieve good hemming with such a conventional roller hemming apparatus, however, the robot must move the hem roller along the edge of the workpiece at a constant distance from the hemming die. The robot is not composed of a perfectly rigid body, and when the hem roller is positioned against the edge of the workpiece, the robot may be deflected by its own resiliency or by a repulsive force exerted by the edge of the workpiece itself. This results in interruption of the pressing force applied by the hem roller and therefore imperfect hemming of the workpiece. Springs have been disposed in the hemming roller support to compensate for the interrupted pressing force, but alone springs have proven inefficient and unpredictable.
Further, the edge of a workpiece to be hemmed may also have a complex configuration. This makes it difficult for a conventional hemming apparatus to achieve adequate hemming of the workpiece.